The theory: Chronic stress leads to weight gain, and chronic stress is at epidemic levels (just like obesity).

The research: Whole books have been written to explain how stress leads to weight gain -- "The Cortisol Connection: Why Stress Makes You Fat and Ruins Your Health -- and What You Can Do About It"; "Fat Around the Middle: How to Lose That Bulge for Good."

Cortisol is often called the "stress hormone" because it's part of the body's fight-or-flight response. It's a good thing when you're being chased by a lion (or chewed out by an angry boss), but many doctors and scientists believe that chronic stress is anything but good.

Studies have shown that cortisol makes people crave rich sweets in the worst way -- and pile on pounds in the worst place, around the middle, putting a body at risk for bad cholesterol, heart attacks and strokes. One study compared women with high waist-to-hip ratios to women with low waist-to-hip ratios and found that the former secreted more cortisol in stressful lab situations and self-reported more stressful feelings.

In 2007, researchers introduced a different notion of how stress is related to weight gain. Their study compared stressed mice (who had to live either in cold cages or with a bunch of mean cousins) and unstressed mice (who, relatively speaking, had the life of Stuart Little before all that bad stuff happened). When fed a high-fat, high-sugar diet, all the mice gained weight, but the stressed mice gained twice as much. The scientists found that a molecule in the stressed mice -- neuropeptide Y -- activated a gene in fat cells, causing the cells to grow in size and number. When that gene was blocked for two weeks, the mice lost 40% of the weight they had gained.

Are we more stressed these days? "I would say that modernity . . . provides more factors that are a source of stress," says Angelo Tremblay of Laval University in Quebec.

Our experts weigh in: Susan Roberts of Tufts University says lots of research shows changes in food preferences for animals under stress. And Dr. Julie Lumeng of the University of Michigan says studies have shown that obese people are less likely than others to be drug addicts or alcoholics -- "the thought being that if you use food to 'soothe your mood,' you will be less likely to need to use alcohol or drugs to 'soothe your mood.' "

Maybe a virus is to blame

The theory: Certain viruses may put people at greater risk of becoming obese.

The research: At least 10 viruses are believed to cause obesity in animals, and two have been tenuously linked to people. Antibodies against one (SMAM-1, which causes obesity in chickens) were found in about 20% of a group of 50 obese people tested in 1992. (Scientists don't know how many non-obese people would have antibodies to this virus as well.) In 2005, in a study of 500 people tested, antibodies to Ad-36 -- a virus that causes symptoms similar to the common cold -- were found in about 30% of obese people, but only in 11% of non-obese people. And in a 2005 test of 89 twin pairs, if one had antibodies and the other didn't, the one who did was generally heavier.

After exposure to Ad-36, chickens, mice, monkeys and rats hardly act under the weather at all, but their body fat increases, sometimes even doubling. Strangely, though, their total cholesterol, "bad" cholesterol and triglyceride levels go down. People who have antibodies for Ad-36 also have better metabolic profiles than people who don't.

It's unknown whether more people are exposed to Ad-36 now than 30 years ago since no one was tested then.

Our experts weigh in: "Obesity due to infection is possible in some people. How big the group is I don't know. . . . Not all obesity is due to viral infection," says Nikhil Dhurandhar of Louisiana State University, who has studied SMAM-1 and Ad-36 and coined the term "infectobesity" for "obesity of infectious origin."

"I think the data are new and emerging, and we just don't know yet," says Dr. Julie Lumeng of the University of Michigan, while Dr. George Bray of Louisiana State University says, "The jury is out on whether these are important in humans."

"I don't buy into these at all," says Susan Roberts of Tufts University.

Maybe the temperature is just right

The theory: Call it the Goldilocks Syndrome. If people rarely get too cold or too hot, but almost always stay "just right" in their temperature comfort zone -- as Americans do, these days, year-round -- they will gain weight.

The research: Environmental temperature can affect weight. A 2002 study of Dutch men found they burned more calories when in a room that was a chilly 61 degrees than when it was a cozier 72 degrees.

That same year, a study with Dutch women found they consumed fewer calories in a too-toasty 81-degree room than in a 72-degree room, because they chose lower-calorie foods to eat. That fits with other science. Studies of people (and pigs) have found they eat less in hot environments.

If the effects seen in short-term experiments hold over time, you'd expect that people who experience sizable temperature variations would be thinner than people who live in conditions that always suit them.

This could only be implicated in the widespread widening of Americans if they're staying in their comfort zone more than they did 30 years ago -- and it seems that they are. Although most homes were heated in the 1970s, better units and better insulation now help keep people in those homes warmer in winter. But greater progress has probably been made in beating the summer heat. Data from the Air-Conditioning, Heating and Refrigeration Institute show that nearly 90% of new homes are now built with central air conditioning, while only about one-third were in the 1970s. And Americans are even more likely to stay cool on the road. Almost every new car has AC now, whereas only about 60% used to back then.

Our experts weigh in: "I am very skeptical about this one," says James Hill of the University of Colorado, and Susan Roberts of Tufts University calls it "small stuff." But Dr. George Bray of Louisiana State University thinks there may be something to it. He notes that temperature affects energy intake and output -- so the more the temperature varies, the more a person's energy balance may bounce around.